1. Field of the Invention
The present invention relates to a wireless communication system, a transmitting device and a receiving device that perform channel estimation by use of pilot signals etc in a multi-carrier transmission method.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) method is adopted as a transmission method in a variety of communication systems and actualizes high-speed data communications at high frequency availability efficiency. The OFDM method is defined as a method of segmenting transmission data into plurality of data, mapping the segmented transmission data onto a plurality of orthogonal carrier waves (subcarriers), and transmitting the data in parallel along a frequency axis.
An examination of a radio frame format employed in such a type of wireless communications is now under consideration in the 3GPP (3rd Generation Partnership Project) etc, and FIG. 12 shows an example of this radio frame format. In the radio frame format illustrated in FIG. 12, each frame is provided at a TTI (Transmission Time Interval) interval, wherein OFDM symbols are allocated in a frequency (an axis of ordinate)-to-time (an axis of abscissa) array within each frame. Further, each frame is generated by a plurality of subcarriers, and each subcarrier consists of 7 symbols along the time axis. Allocated to each subcarrier are a common pilot channel (Common Pilot Channel), a dedicated pilot channel (Dedicated Pilot Channel) and other channels (Other Channels which will hereinafter be referred to as data channels), respectively. A pilot symbol common to all users is allocated to the common pilot channel, a dedicated pilot symbol assigned corresponding to each of predetermined users is allocated to the dedicated pilot channel, and data symbols are allocated to the data channels.
In a receiving device utilizing this type of frame format, influence by fading is compensated by a propagation characteristic value (a channel estimation value) etc estimated by use of this pilot symbol. In this case, the fading influence is different for every symbol, and hence the receiving device is required to interpolate the channel estimation value on the frequency axis and on a time axis on the occasion of demodulating each symbol. For instance, the example in FIG. 12 illustrates that the channel estimation value existing in each symbol position is linearly interpolated by use of a pilot symbol CP-1 and a pilot symbol DP-1, and symbols 1-A and 1-B are demodulated based on the thus-obtained channel estimation value.
It should be noted that the following documents disclose the conventional arts related to the invention of the present application. The conventional art documents are “Japanese Patent Application Laid-Open Publication No. 2003-032146” and “Technical Specification Group Radio Access Network, “Physical Layer Aspects for Evolved UTRA (Release 7)”, 3rd Generation Partnership Project, 3GPP TR 25.814 V1.0.1, Nov. 2005, p. 22-24.”
It is known that in the radio frame format described above, the number of the pilot symbols and positions of the pilot symbols to be allocated largely contribute to communication performance such a received error rate. For example, it follows that a transmission rate decreases though channel estimation accuracy is improved in the case of increasing the number of the pilot symbols, and the received error rate is deteriorated while the channel estimation accuracy is lowered in the case of decreasing the number of the pilot symbols. Further, the fading influence depends on a propagation environment etc, and hence, even when using the radio frame having the same pilot symbol allocation, the channel estimation accuracy based on this pilot symbol allocation changes corresponding to the propagation environment etc.